Component alignment is of critical importance in microoptical systems and especially semiconductor and/or MOEMS (microoptical electromechanical systems) optical system manufacturing. The basic nature of light requires that light generating, transmitting, and modifying components must be positioned accurately with respect to one another, especially in the context of free-space-interconnect optical systems, in order to function properly and effectively. Scales characteristic of optical semiconductor and MOEMS technologies can necessitate micron to sub-micron alignment accuracy.
Consider the specific example of coupling light from a semiconductor diode laser, such as a pump or transmitter laser, to single mode fiber. Only the power that is coupled into the fiber core is usable, and the coupling efficiency is highly dependent on accurate alignment between the laser output facet and the core; inaccurate alignment can result in partial or complete loss of signal transmission through the optical system. Moreover, if polarization-maintaining fiber is used, there is an added need to rotationally align the fiber relative to the laser to maintain the single polarization characteristic of the output signal.
Other more general examples include optical amplification, receiving and/or processing systems. Some alignment is typically required between an optical signal source, such as the fiber endface, and a defector. In more complex systems including tunable filters, for example, alignment is required not only to preserve signal power, dynamic range, but also to yield high quality systems through the suppression of undesirable optical modes within and without the systems.
In the example of a tunable filter system, light, typically supplied by fiber, is injected into an optical train, which includes a tunable filter, such as Fabry-Perot (FP) tunable filter. The launch characteristics of the light into the FP filter cavity determine the side mode suppression ratio (SMSR) of the system. This ratio, in part, dictates the quality of the system. If light is launched into the filter at the wrong position or with the wrong spot size, higher order modes are excited in the filter, degrading the system""s SMSR. Typically, filter train alignment is employed to extract the highest possible SMSR.
Generally, there are two types of alignment strategies: active and passive. Passive alignment relies on the precision placement of the optical components relative to some physical reference. For example, registration or alignment features can be fabricated directly on the optical components, such as the optical elements or element mounting structures, as well as on the platform to which the components are to be mounted. The components are then mounted and bonded directly to the platform using the alignment features. Other techniques rely on vision and/or metrology systems. Contrastingly, in active alignment, an optical signal is transmitted through the components and detected. The alignment is performed based on the transmission characteristics to enable the highest possible performance level for the system.
In the context of commercial volume manufacturing, selection between active and passive alignment, or some mix of the two, is determined based on the quality of part needed. Lower cost, lower performance devices are typically manufactured with entirely passive alignment strategies, whereas the manufacture of high performance devices typically involves at least some active alignment.
In the manufacture of optical systems, it is typically possible to passively align two optical components, such as components including lenses. In micro-optical trains, where beam diameters can be less than one millimeter and are usually less than 500 micrometers, alignment accuracies of 10 micrometers are possible by utilizing jigs, optical component templates and/or registration/alignment feature systems.
In other situations, such as carrier-class systems and/or systems that have tunable optical filters, alignment tolerances of better than 5 micrometers are common. Moreover, in some implementations, submicrometer alignment tolerances are required, and even sub-100 nanometer tolerances can be required to achieve high side mode suppression ratios, for example, in tunable filter systems or when maximizing coupling efficiency in high power laser systems. In such optical systems, the alignment tolerances are more rigorous than that obtainable with conventional passive alignment techniques. As a result, active alignment is required.
In either case, whether active or passive alignment or a combination of active and passive alignment is used with the result being that the optical component has been placed or otherwise located, the component must be affixed to its carrier, typically called a submount or optical bench. While an adhesive such as epoxy can be used, solder bonding or laser welding is typically the preferred modality of affixing the component, because of advantages associated with a well-understood behavior such as long-term stability over wide temperature ranges.
The bonding process can require the physical movement of the bench with the placed optical component and/or removal from placement/alignment systems. For example, the best solder bonds are typically achieved in solder reflow ovens, which provide a well-controlled reflow environment. Laser welding devices must have a clear path to the areas of the optical component where the welds are to be produced. This requirement can create contention with the placement strategy used.
The present invention is directed to a magnetically-assisted fixturing process for optical components on a bench. Basically, the process and associated system alignment station utilize precision placement of an optical component on a bench. This placement can be done either entirely passively, actively, or using a combination of active and passive alignment. The optical component is then held on the bench using a magnetic field. Thus, the optical component is maintained in a stable relationship with respect to the bench, especially after it has been aligned. The optical component is then affixed to the bench, typically by a solder bonding process. Alternatively, other bonding processes can be used, such as epoxy bonding or laser welding. In one implementation, the magnetic fixturing is maintained during the process of affixing or bonding the optical component. Thus, in one example, the optical bench can be transported to a solder reflow oven while the optical component is held on the bench via the magnetic fixturing.
Moreover, in some implementations, the magnetic field is maintained during the solder reflow process. This can restrict both lateral movement of the component, while the solder is molten, and also yield a final solder layer between that component and the bench that has a uniform thickness from component-to-component. This can be an important advantage of the system since it yields a uniform y-axis, i.e., orthogonal to the plane of the optical bench, optical axis height.
In general, according to one aspect, the invention features a magnetically-assisted fixturing process for optical components to a bench. The process comprises placing at least one optical component on the bench, and then holding the optical component on the bench with a magnetic field. The optical component is then affixed to the bench.
In one implementation, the bench is constructed from a non-magnetic material. The optical component, however, comprises ferromagnetic material. The magnetic field is oriented to pull the optical component against the bench.
In the current implementation, the optical component comprises an optical element, such as a lens or tunable or fixed filter. A mounting structure is used to support this optical element on and above the optical bench. The mounting structures are made from nickel or an alloy including nickel in current implementations.
Depending on the implementation, permanent magnets or electromagnets are used to generate the magnetic field. Electromagnets have an advantage in that their electric field can be modulated without removing the magnet structure. Thus, the strength of the magnetic field can be temporarily reduced while optical component is placed or during replacement of the optical component in which its alignment is improved.
In one embodiment, an active alignment process is used for the optical component. Specifically, an optical signal is generated that interacts with the optical component. For example, the optical signal may be transmitted through the optical component or reflected by the optical component. The optical signal is then subsequently detected. The optical component is then positioned relative to the optical system in response to the detected optical signal.
In any case, once the optical component is placed, its position is preferably confirmed. If the alignment is inadequate, a repositioning or realignment step can be performed. According to the preferred embodiment, once the optical component has been placed, the optical bench is removed from the alignment station and installed in an SRO for solder reflow. If permanent magnets are used, they can be easily kept with the bench during the solder reflow process to maintain the positioning of the optical component. Alternatively, the magnetic field is removed during the reflow process.
Alternatively or in addition to: active alignment, passive alignment techniques are preferably employed to at least initially locate the optical component on the bench. For example, bench registration features such as raised bench features can be used and the optical component abutted against these features. Alternatively, marks on the bench can be used for the optical component placing process.
In general, according to another aspect, the invention also features an optical system alignment station. The station comprises an optical system chuck that secures the optical system. An optical component alignment system supports an optical component in proximity to or on the optical system bench and enables orientation of the optical component relative to the bench. A magnetic field generator then generates a magnetic field that pulls the optical component into engagement with the bench.
In one embodiment, an active alignment system is used. Specific an alignment signal generator generates an optical signal that interacts with the optical component. An optical signal detector then detects the alignment signal after interaction with the optical component. The optical component alignment system then positions the optical component in response to the alignment signal detector. Depending on the configuration of the optical signal, the optical signal generator can be a laser system that is part of the optical system. Alternatively, the alignment signal generator can be a separate system that generates an optical signal that is used during the alignment process. Further, the alignment signal detector can be part of the optical system or a separate detector that is inserted into the optical signal only for the alignment process.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.